[单选题]压强和温度会引起密度的变化，假定其他条件相同的情况下，下列相关的说法不正确的是()。
A.氧气瓶中的氧气用掉一部分后，体积不变，密度变小
B.给自行车胎打气，体积不变，质量变大，密度变大
C.对绝大多数物质来说，温度升高，密度变小，则水在0℃时的密度大于4℃时的密度
D.温度计内的水银随温度升高而密度减小引起体积变大

When you look at the sky on a clear day, it seems as if the Earth is covered by a blue dome (圆盖). The sky is blue (41) what happens to sunlight as it enters the Earth’s atmosphere. Sunlight (42) light rays of all lengths. The longer rays make up red light and the shorter rays make up blue light. (43) sunlight enters the Earth’s atmosphere, it strikes the tiny (44) of air and is scattered. The scattered light is made up mostly of violet, blue, a little green, and a little yellow and red. The (45) of these colors is sky blue.
Sometimes (46) sunset the sky looks red. The sunlight is coming toward you from a lower (47) in the sky. The lower the sun, the (48) is the path that the light rays must travel through the air. The shorter blue rays (49) immediately when striking molecules of air. The sunlight that you see (50) the blue and violet rays that the air has scattered. As the sun gets (51) <
A. because
B. if
C. unless
D. before